I just filled out the 2018 w4. I’m married filing joint. We claim 6. Can we split that He claim 3 & I 3? Also we have to wh additional tax do we only do that on 1 check?

Like many tax items, it depends.  What depends here is how you want to get to your final goal.  Next year, Married Filing Joint, roughly 70K of income, means that you'll have 46K of taxable income, assuming the 24K standard deduction.  The first 19,000 of your taxable income is taxed at 10%, and with the new tax brackets, the remaining amount is at 12%.  That tax amount comes to around 5140 (we'll say 5200 to round up).  If your child is under 17, then you will receive a 2,000 credit, bringing your tax liability to 3200.  This means that for each quarter, you need to be paying in roughly 800 between the two of you to make a balance near zero.  Keep in mind that these figures do not account for additional credits or liabilities that you may have.  Also, if your dependent is 17 or above, the 2,000 credit is reduced to 500, and the tax liability still due is 4700.  

That is your target number; how you get there is up to you.  I don't personally recommend the "extra withholdings" option when you both are about in the same income category,  It is easier to say married with 2 allowances each or something similar.  But W-4 withholding options are truly your choice.  The IRS is less concerned about who is contributing what as much as the fact that you are being withheld for taxes timely and in your final amounts you do not owe more than 1,000 of tax when you file your return.